1. Field of the Invention
The invention relates to stabilized platforms that maintain a fixed orientation with respect to inertial axes for use on moving vehicles.
2. Description of the Prior Art
Mechanically gimballed stabilized platforms are known in the prior art for maintaining a fixed orientation with respect to inertial axes. Such platforms are stabilized by a stabilization data source, such as one or more gyros and generally stabilize pointing devices or systems used aboard moving vehicles such as ground vehicles, aircraft, marine vessels and space craft from the movement of the vehicle. Such pointing systems include radar antennas, optical sights, cameras, satellite antennas, and the like. Isolation of the movement of the vehicle from the pointing device is required, since the movement would result in errors in the line of sight of the device, creating blurring or even rendering the device inoperative. For example, in an airborne, ground mapping radar, the motion of the aircraft tends to blur the ground targets and to lower the perceived resolution of the radar picture. The vehicle motion would also cause the targets to appear at different locations from sweep to sweep, making identification of targets difficult. Thus, stabilized platforms are utilized on moving vehicles such that the motion of the vehicle does not interfere with the gathering of data by a pointing device. The pointing device is generally moved relative to the stabilized platform to point at targets.
Traditionally, stabilization is accomplished utilizing a mechanical gimbal system that is separate and in addition to the gimbals that move the pointing device relative to the platform. The platform gimbal system duplicates the gimbal arrangement of the stabilization data source, such as a gyro. Typically, a mechanically stabilized platform has two stabilization axes with a third axis for rotating the pointing device relative to the platform. The pointing device itself may require two axes relative to the platform, one for azimuth and the other for elevation, thus unduly increasing the total number of gimbals required for the system and decreasing overall system reliability.
With the prior art mechanically gimballed platform, the order of precession of the gimbals in the stabilization gyro dictates the order of precession of the axes of the platform. This is because a gimbal is always referenced to the gimbal in which it is mounted. For example, in a two axis system where the gyro gimbals are arranged as pitch inside of roll, the stable platform gimbals must be constructed with pitch inside of roll. If a third axis is desired for rotation of a pointing device with respect to the platform, the rotation of the third axis must follow in the precession order of the platform axes. This limitation results in non-optimum system designs. In the example of the airborne, ground mapping radar system, the precessional order of the axes results in the antenna hanging down on a lever arm mounted to the platform and sweeping over a large volume relative to the aircraft as the antenna rotates and the aircraft rolls and pitches. Generally this is undesirable, since the large swept volume required for the moving antenna conflicts with space limitations normally associated with aircraft. The large antennas required in narrow beam, high resolution radar systems further exacerbate the problem.
In order to overcome the above-described limitation, the prior art utilizes push/pull linkages or rods with mechanical gearing, or other mechanical linkages, to translate the motion of one axis through the others. For example, in a radar system, the azimuth sweep of the antenna may be translated through the roll and pitch axes by push rods to provide the appropriate rotary motion of the antenna on the stable platform. Although the gimbals are arranged in the order of azimuth, roll and pitch, the linkages cause the antenna pedestal to behave as though the gimbals were arranged in a different order. Thus, in such systems, the push/pull linkages effectively allow the pedestal to perform the stabilization of the antenna as if the gimbals were arranged in the order of roll, pitch, and then azimuth. Mechanical linkages suffer from low reliability, difficulty in assembly, and wearout of the mechanisms. Although these mechanical gear and linkage arrangements reduce the swept volume of the antenna and provide proper stabilization, component wear over time tends to decrease the overall system reliability.
Thus, traditional antenna pedestal systems utilize a two-axis stabilized platform about which the spinning or rotational azimuth scan occurs. Not only does this result in an increase in swept volume of the rotating antenna, but the drive systems usually comprise geared rotational arrangements resulting in poor lifetime reliability. Furthermore, an additional antenna degree of freedom, such as elevation, requires yet another gimbal set thereby increasing complexity and expense and decreasing system reliability.